Generally, a semiconductor memory device is provided with an external voltage VDD and a ground voltage VSS from an exterior and generates and uses an internal voltage required for internal operations. The voltages required for internal operations of a memory device are a core voltage Vcore supplied to a memory core region, a high voltage VPP used when overdriving or driving word lines, a back bias voltage VBB supplied as a bulk voltage of NMOS transistors of a core region and so on.
Here, the core voltage Vcore may be supplied by reducing the external voltage VDD input from an exterior to a predetermined level. However, the high voltage VPP has a higher voltage level than the external voltage VDD and the back bias voltage VBB maintains a lower voltage level than the ground voltage VSS supplied from an exterior. Therefore, in order to supply the high voltage VPP and the back bias voltage VBB, a charge pumping circuit is required to supply charge for the high voltage VPP and the back bias voltage VBB.
There are two types of conventional high voltage VPP pumping circuits, one using a high voltage VPP pump 2 of doubler structure, as shown in FIG. 1A, and the other using a high voltage VPP pump 3 of tripler structure, as shown in FIG. 1B.
The high voltage VPP pumping circuits as shown in FIGS. 1A and 1B are driven in response to a power up signal PWRUP which is enabled to a high level when the external voltage VDD level is over a predetermined level in an initial operation section of a semiconductor memory device.
Generally, since the high voltage VPP pump 2 of doubler structure has a maximum pumping efficiency reaching 50%, it is advantageous as compared with the high voltage VPP pump 3 of tripler structure having a maximum pumping efficiency of 33% in the aspect of efficiency.
On the other hand, the high voltage VPP pump 3 of tripler structure is advantageous in the aspect of reliability. This is because the high voltage VPP pump 2 of doubler structure can pump the high voltage VPP to a level twice as high as the external voltage VDD, whereas the high voltage VPP pump 3 of tripler structure can pump the high voltage VPP to a level three times as high as the external voltage VDD.
However, a conventional high voltage VPP pumping circuit selectively using one of the high voltage VPP pump 2 of doubler structure and the high voltage VPP pump 3 of tripler structure is unable to satisfying both reliability and efficiency.